1. Field of the Invention
The present invention relates to an improved technique of measuring power provided by a polyphase current/voltage source, the power being supplied to an utilization system.
2. Background of the Invention
Apparatus for measuring electrical power supplied by an A-C polyphase power source to a load have long been available in the prior art. Typical of these prior art apparatus are those disclosed in the following United States patents:
U.S. Pat. No. 3,975,682 discloses a watt/watthour meter wherein a current proportional to instantaneous power is obtained from a current voltage multiplier circuit, the current being integrated to provide a watthour output signal. A charge compensation circuit is provided to integrate one of the watt signal currents in either direction corresponding to power in or power out to minimize the error in the integrating section of the watt/watthour meter; U.S. Pat. No. 3,976,942 is similar to that of U.S. Pat. No. 3,975,682 with the pulse width modulator utilized in the multiplier modified to provide two current outputs instead of a voltage to minimize system errors in power measurements; U.S. Pat No. 4,055,804 is similar to U.S. Pat. Nos. 3,975,682 and 3,976,942 except that the power related current output signal of the modulator is amplified by a current-to-amplifier instead of a voltage amplifier; U.S. Pat. No. 4,066,960 relates to an electronic kilowatt hour meter having internal error correction for metering electrical power and energy consumption in a single phase or polyphase electrical system. A pulse width modulated signal having a pulse duration proportional to the amplitude of the voltage in the system is multiplied by a signal which is proportional to the current in the system. The output of the multiplier is integrated to provide a signal which is proportional to the energy consumed in the system; U.S. Pat. No. 4,056,775 discloses a technique for metering kilowatt hours in an electrical system by generating pairs of analog signals representing current and voltage variables, the basic circuitry for metering kilowatts and kilowatt hours being similar to that disclosed in U.S. Pat. No. 4,066,960; U.S. Pat. No. 4,058,768 discloses a two-way electronic kilowatt meter for metering electrical power and energy consumption in a single phase or polyphase system. Pairs of analog signals representing current and voltage variables are derived, each pair of analog signals being multiplied together to derive partial instantaneous power signals. These signals are summed and integrated to provide a signal which is proportional to the total energy being imported or exported with respect to the electrical system; U.S. Pat. No. 4,217,546 discloses an electronic energy consumption meter and system which includes means for automatically correcting for overall system signal and voltage errors and offset errors created by circuits in the system; U.S. Pat. No. 4,485,343 describes an electronic watthour and watt meter circuit providing both analog and digital outputs and having an automatic error correction and utilizes first and second input transformers for developing first and second signals proportional respectively to the voltage and current being supplied from a source of electrical energy to a consumer. The first and second signals are supplied to a multiplier circuit for developing pulse-width-modulated and amplitude modulated product output signals representative of instantaneous watts. Automatic error correction is provided wherein automatic addition/subtraction averaging of system offset error signals during operation is accomplished; U.S. Pat. No. 4,495,463 discloses an electronic watt and kilowatt hour measuring circuit and uses a current transformer sensor for sensing alternating load current and a voltage transformer for sensing the alternating current voltage supplied from a source of alternating current electric current energy being monitored; U.S. Pat. No. 4,535,287 discloses an electronic watt-watthour meter circuit which comprises first and second transformers for developing first and second voltage and current indicating signals for supply to a multiplier circuit that multiplies the two signals together and derives an output analog product signal representative of the instataneous power being supplied by a source of electric energy; U.S. Pat. No. 4,456,878 describes an electronic watthour meter which includes a pulse width modulation circuit for converting a voltage signal proportional to the load voltage across the power supply lines to a pulse width duty cycle signal and a current to voltage converter for converting a current signal proportional to the load current across the power supply lines to a voltage signal. The multiplication circuit provides a product of the voltage signals as a signal proportional to the instantaneous power consumption of the power supply lines; U.S. Pat. No. 4,408,283 relates to a power evaluating apparatus such as power transducers utilizing a time division multiplication approach and providing a phase adjustment which adjusts for reactive power and energy measurement; U.S. Pat. No. 4,303,881 discloses a device for providing multiple functions including the measurement of power which is done by measuring the voltage and current from line voltage and current voltage samples and applying them to a scaled multiplier, the output of the multiplier providing an indication of true power; U.S. Pat. No. 4,118,787 discloses an error correcting device for a multiplier of the time-division multiplication type comprising a modulator, a switch and a filter; and U.S. Pat. No. 4,333,118 discloses a load indicator for AC motors which has an analog multiplying circuit for producing a signal proportional to the product of the current of the motor and the voltage thereof. From this signal a signal is subtracted which is proportional to the absolute value of the motor current, the average value of the differential signal being a measure of the load of the motor.
Although the aforementioned patents disclose various apparatus/techniques for measuring electrical power, concepts for reducing the cost/complexity thereof have not been specifically addressed. In particular, existing methods of measuring power supplied by a multiplier line (3 phase) power source to a load typically requires in the measuring circuit at least three separate RMS to DC chips to provide a measure of apparent power supplied over each of the three power input lines and three separate chips, such as low pass filters, to provide output signals representing the actual power consumed in the system. The necessity of thus using these additional chips to provide a power measurement adds to the cost of the overall apparatus. In the competitive business of providing power measuring apparatus, the vendor which can provide a device which measures power accurately and at a reduced cost would have an advantage over his competitors. Thus what is desired is to provide an apparatus which provides an indication of the actual and apparent power consumed by a system which is provided power by a polyphase distribution system, the measurement being accurate and accomplished at a cost which is significantly less than those of the prior art apparatus.